There is known from the prior art, in particular from EP Patent No. 1 484 009, a portable instrument provided with an optical device for measuring a physiological quantity. This document describes in particular, as shown in FIG. 6, a portable instrument including a light source 61 (for example a light emitting diode, i.e. LED, or any other suitable device) coupled to a control circuit 71, whose operation is controlled by a central processing unit 70, such as a microprocessor or microcontroller. This central unit 70 is further interfaced with a display device 73 (of analogue and/or digital type), storage means 74 (RAM, ROM, EEPROM FLASH or the like) and a clock system 75, for properly clocking the operation of central unit 70 and its peripheral components. This clock system 75 can further perform the conventional clock functions of a timekeeper.
The central processing unit is also coupled to a circuit 72 dedicated to detection of the desired physiological quantity measurement, for example the heart rate or the level of oxygen in the blood, the functions of this circuit being able to be integrated with those of central processing unit 70. This circuit extracts data relative to the physiological quantity from optical signals detected by the associated photoreceptor(s). In this case, a first photoreceptor 62 is coupled to detection circuit 72 by amplification and, if necessary, filtering means 63. Data relating to the desired physiological quantity is transmitted to central processing unit 70, particularly in order to be displayed on device 73 and/or stored in storage means 74 for subsequent consultation.
According to one embodiment that can be envisaged, shown in FIG. 7, an amplifier in series with a high pass filter is provided for making the amplification and filtering means at the output of the photoreceptor. The external signal IN is first of all amplified through an amplifier circuit 81, performing a current voltage converter function. In addition to amplifier circuit 81, the first stage comprises a high pass filter 82 that can be made for example by means of a Sallen Key type high pass filter with a finite gain amplifier. Thus, the ambient component of external signal IN, which is not modulated, is removed through the high pass filter 82. An amplified IN1 signal, comprising only the useful component of the detected signal, is transmitted at the output of the amplification and filtering means to the next stage for conditioning of the signals.
Nonetheless, within the scope of the present invention, it has been demonstrated that this solution is not optimal insofar as the ambient signal that is a component of the received external signal IN is also amplified through amplifier 81, which considerably limits the usable gain range of the amplifier to prevent the latter becoming saturated. The noise over signal ratio would then be less advantageous at the input of the following stage.
Moreover, this portable instrument and more particularly the conditioning circuit for conditioning the signals received by the optical sensor prior to processing by the central processing unit, has certain drawbacks, particularly in terms of the space occupied in the portable instrument and in terms of power consumption. Indeed, in this portable electronic instrument, two major concerns are typically the available space and power consumption which are both limited. A conditioning circuit like that presented in the prior art has a non-optimum occupied surface area in that it uses a certain number of discrete components to perform the functions of amplification, filtering and detection. Moreover, each of these functions has non-optimised energy consumption because of the use of numerous operational amplifiers.